The ‘compact city’ concept is prominent in contemporary planning policy debates about ideal urban forms. However, the property of compactness itself is not well defined, and is sometimes confused or conflated with density. This chapter develops a new geometric interpretation of compactness with specific indicators—relating to diameter and perimeter—that can capture this property in the urban context. The chapter demonstrates these compactness indicators first by application to theoretical geometric shapes and then a range of English urban areas. The chapter reflects on the interpretation of the core concept of compactness, and suggests additional indicators such as ‘built compactness’ and ‘population compactness’.